It is known that special color effects can be achieved by incorporation of carbon black into pigments. However, incorporation of carbon black into masses being pigmented is difficult, since the carbon black must be in finely dispersed form. Adsorption of carbon black on freshly precipitated iron hydroxide has been attempted and described in British Patent Specification No. 1,178,050. Incorporation of carbon black into flaky pigments has been described, e.g., in published German Patent Specification No. 1,165,182, as a way of obtaining particularly attractive color effects in lustrous pigments.
Known processes in which carbon black is produced by pyrolysis of organic compounds have several disadvantages:
(a) high capital outlay for apparatus, since the pigment being coated with carbon black is heated, for example, with hydrocarbons at temperatures between 700.degree. and 1000.degree. C. PA1 (b) the operation requires exclusion of oxygen, which is technically difficult. PA1 (c) upon heating above 700.degree. C., interference colors of lustrous pigments change, depending on the period of calcination and temperature. PA1 (d) pigments thus-produced contain free carbon black which, when the pigment is suspended, separates and becomes disturbingly noticeable. PA1 (e) according to German Patent Specification No. 1,165,182, only pigments are formed in which carbon black is on the surface and fills in intermediate spaces between TiO.sub.2 grains in a layer thereof on the mica. PA1 (a) precipitating an insoluble metal hydroxide or oxide or bismuth oxychloride in an aqueous suspension of a particulate substrate and carbon black of particle size below 1 mn., thereby forming a coating on the substrate.
Thus, there is a need for methods of preparing stable, commercially useful, easily handled, carbon black-containing pigments, and for providing lustrous pigments which, because of their carbon black content, have special color effects.
This problem is solved by a new process for fixing carbon black particles on any desired substrate.